1. Field of the Invention
The present invention relates to stratified cobalt enriched cemented carbide cutting tools having adherent coatings of titanium carbonitride and process for deposition of those coatings.
2. Description of the Related Art
Cemented carbide cutting tools are widely used in metal cutting applications owing to their unique properties of high hardness, toughness, strength, and wear resistance of cemented carbide materials can be further improved by applying a thin layer of a coating of a refractory compound such as titanium carbide, titanium nitride, aluminum oxide, and combinations thereof. Such coatings have widened the application range of cemented carbide tools.
Advances in metal working equipment manufacture and economic necessities of higher productivity have combined to put greater demands of improved performance on the cutting tool materials.
Tungsten carbide based sintered alloys are used as substrate material for the coated cutting tools because of the high mechanical strength, hardness and good wear resistance. As a substrate material, the tungsten carbide based alloy has higher toughness than other materials such as titanium carbide based sintered alloys, titanium nitride based sintered alloys and alumina-based ceramics. However, when the tungsten carbide based substrates are coated with titanium carbide by chemical vapor deposition techniques, a brittle layer of a complex carbide, known as the eta phase, tends to form around the tungsten carbide grains. The brittle layer acts as a site initiating the propagation of cracks so that the cutting inserts with this deleterious complex exhibit a marked reduction in strength.
The deleterious effect of eta phase is described in the papers by Bhat, Cho and Woerner (Relationship between metal-cutting performance and material properties of TiC-coated cemented carbide cutting tools, Surface and Coatings Technology, Vol. 29, 1986, pp. 239-246; the role of interface development during chemical vapor deposition in the performance of TiC-coated cemented carbide cutting tools, Journal of Vacuum Science and Technology, Vol. A4(6), 1986, pp. 2713-2716).
Another problem encountered during vapor phase deposition is the diffusion of cobalt to the surface and into the TiCN coatings. One technique to avoid these problems is not to use titanium carbide. Several practitioners of the art resort to depositing a film of TiN on the surface first.